Optical lens blanks for producing spectacle lenses are often produced, without cutting, from plastic or silicate glass and preferably have a circular lens circumference. Lens blanks made of plastic are generally obtained by moulding or injection moulding, and lens blanks made of silicate glass are generally produced by immersing heated glass rounds into moulds. The lens blanks often already have a first lens surface (in most cases the front) with an optical effect, that is to say the first lens surface does not have to be reworked. An opposite second lens surface (in most cases the rear) is later reworked by means of CNC machinery.
The second lens surface is reworked in most cases according to a prescription that describes the individual visual defect of a prescription holder. To rework the second lens surface, a prescription surface is calculated on the basis of the prescription and of the geometry of the first lens surface. This prescription surface is worked out from the second lens surface and then, together with the first lens surface, forms a prescription lens which corrects the visual defect of the prescription holder. To this end, mechanical working is in most cases first carried out, for example, by turning, milling and grinding. The mechanical working is then followed by a polishing operation. In this operation, the roughness of the surfaces is reduced to such an extent that light is no longer improperly scattered and the lens is thus transparent. A spectacle lens is finally obtained by adapting the circumference geometry of the lens blank to a spectacle frame, often referred to as “edging.”
Particularly for the correction of presbyopia, it may be necessary for the spectacle lens to have several focuses. The crystalline lens curves to different extents depending on the distance of the object. In presbyopia, this elasticity is limited: the crystalline lens hardens and becomes thicker. This has the effect that the visual defect of the eye for objects at a short distance from the eye (near vision) is different than the visual defect for objects at a far distance (distance vision). To be able to see objects clearly at all distances from the eye, the focus of the eye has to be corrected, depending on the distance of the object from the eye, by a defined focus of a lens. By producing the prescription surface, it is possible to configure a lens in such a way that it has a plurality of focuses in different zones of the spectacle lens. The zone within the spectacle lens is changed by a modification of the viewing direction.
If a spectacle lens has two different focuses, generally one for distance vision, and one for near vision in the lower area of the spectacle lens, then the lens is designated as a bifocal lens. Lenses of this kind are, in most cases, distinguishable by a visible transition between the two areas. If the focus frequently changes to another as the viewing direction changes, in particular, from up to down, the lens is designated as a multifocal lens. In this case, the focus can also be continuously variable.
Most lens blanks for producing spectacle lenses have both a spherical front and a spherical rear. Spherical is to be understood as meaning that the surface is a segment of a sphere or that all the points on the surface lie at a common distance or radius from a center point of the sphere.
Moreover, EP 1 990 676 A2 discloses a lens blank which is asymmetrical and in which only the rear is intended to be reworked. The front of the lens blank is formed by two partial surfaces. While the main surface is a sphere, a toric surface is arranged decentrally in the main surface. This toric surface has a lesser curvature than the main surface and is intended to be arranged subsequently in the lower area of the spectacle lens, in particular, in the area of the spectacle lens that is intended to correct the visual defect in the near-vision range. Toric surfaces or toric lenses serve not only to correct short-sightedness or far-sightedness but also to compensate for astigmatism. Their design is more complicated. On a prescription for toric lenses, the strength is noted in three parts. The individual parts are designated as sphere (sph.), cylinder (cyl.) and axis.
A disadvantage of EP 1 990 676 A2 is that the prismatic effect in the horizontal direction is not taken into consideration. The resultant blurring in the viewing directions to the left and right is an inconvenience to the person wearing the spectacles and, e.g., in the case of a vehicle driver who glances over his shoulder through such spectacles, it also poses a danger to himself and to others. This blurring also means that people wearing such spectacles often also suffer from nausea, headaches and feelings of dizziness. Moreover, the aesthetics of the spectacle lens are impaired, since the transition from the spherical surface to the toric surface on the front of the lens can be seen, in particular, through reflection of light.